The present invention relates to reflectometer measurements.
Optical reflectometer measurements are applied for testing fiber optic cables in today""s fiber optic network, and described in detail e.g. by the inventor in chapter 11 of the book xe2x80x98Fiber Optic Test and Measurementxe2x80x99 by Derickson Dennis, 1998, ISBN 0-13-534330, in U.S. Pat. Nos. 5,589,933, or in 5,963,313.
It is an object of the invention to provide an improved reflectometer measurement. This is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
According to the present invention, a reflectometer is provided for measuringxe2x80x94in response to a stimulus signalxe2x80x94return signals, reflected and/or backscattered in a network to be measured. A receiver of the reflectometer is adapted to receive the return signals at different receiver sensitivities.
In operation, at least two different reflectometer measurements can be conducted either after each other or alternatively: a distant range measurement with higher receiver sensitivity for measuring return signals from a more distant range of the network, and a close range measurement with lower receiver sensitivity for measuring return signals from a closer range of the network.
During the distant range measurement, the operation of the receiver will be temporarily disabled (or at least be suppressed) during such operation modes of the receiver, wherein return signals resulting from closer ranges of the network would cause the receiver to generate substantial spurious signals, which lead to interference when the receiver will receive return signals resulting from more distant ranges of the network. Without such disabling or suppression, the generated substantial spurious signals would superimpose with the received return signals resulting from the more distant ranges of the network.
The term xe2x80x98spurious signalsxe2x80x99 as used herein shall mean unwanted signals not resulting from stochastic noise but may be caused e.g. by dynamic recovery processes as result to e.g. thermal increase in the receiver resulting from the closer range return signals. Such closer range return signals generally have significantly higher power than the return signals from the more distant range of the network. However, such high power return signals received with the higher receiver sensitivity (in the distant range measurement) can lead to a significant temperature rise of the receiver which can cause spurious signals in the receiver resulting from dynamic recovery processes once the heating up stops or is decreased.
It is to be understood that while stochastic noise can generally be extracted to a certain extend from the measured results, spurious signals can hardly be detected as such and will therefore be regarded as (valid) signals thus adulterating the measurement. In particular spurious signals resulting from dynamic recovery process generally exhibit exponential characteristic xe2x80x98matching wellxe2x80x99 with expected reflectometer measuring results, and become therefore not transparent as measuring faults.
Due to the reflected and/or backscattered nature of the return signals, the return signals from the closer range will appear earlier at the receiver than the return signals from the more distant range of the network. That means that the higher power closer range return signals will first heat up the receiver, or components of the receiver, which will then later cause spurious signals when the lower power distant range return signals appear. Due to the significant power differences between return signals from closer and distant ranges, the thus xe2x80x98inducedxe2x80x99 spurious signals can significantly disturb the distant range measurement. The invention, however, avoids or at least reduces the generation of such spurious signals by disabling (or at least suppressing) the operation of the receiver during such times of the distant range measurement, when the closer range return signals with higher power appear, which would cause such recovery processes and thus the spurious signals.
Since the return signals from the closer range will appear earlier at the receiver than the return signals from the more distant range of the network, the invention can make use of that effect by disabling (or at least suppressing) such higher power closer range return signals preferably until the lower power distant range return signals appear.
The differentiation between such higher power closer range return signals (to be disabled or suppressed) and the lower power distant range return signals can be accomplished by e.g. establishing trigger or threshold values (e.g. for the power) of the return signals, by defining an appropriate time interval, or by experience resulting e.g. from previous measurements.
In a preferred embodiment, the disabling (or at least suppressing) of the operation of the receiver is accomplished by masking the return signal, e.g. by providing a switch, attenuation or shutter element in the signal path of the return signal of the receiver, so that the return signal will only reach partly, attenuated or not at all the receiver.
In another embodiment, the receiver can be switched off for disabling (or at least suppressing) the operation thereof. Preferably in case the receiver comprises a photo diode (e.g. an avalanche photo diode APD), this photo diode will be switched off by modifying its bias voltage (as well known in the art).
In case the disabling (or at least suppressing) of the operation of the receiver leads to transient behaviors thereof, such transient behaviors have to be taken into account when interpreting the measuring results. This can be done e.g. by allowing for a time interval for the transition from low sensitivity to high sensitivity state that is afterwards discarded in the measurement result.
A complete measurement result can be obtained by combining the results of the at least two reflectometer measurements as disclosed e.g. in the aforementioned book xe2x80x98Fiber Optic Test and Measurementxe2x80x99.
In a preferred embodiment employing an optical time domain reflectometer (OTDR), the measurement hardware is set to a constant gain during each trace acquisition. The gain is then changed during the trace acquisition. This way the receiver is set to an insensitive state first, when high power signals are likely to occur, and then set to high sensitivity when small signals only reach the receiver because of the attenuation of the optical fiber. The advantage is that, with proper implementation, high power levels won""t generate a high temporal power dissipation in the photo diode, which otherwise would lead to an undesirable thermal recovery process.
A standard approach for increasing the performance, e.g. the measurement range of OTDRs, is the use of laser diodes with very high output power. This is a rather straightforward way, since the fiber response signal is proportional to the probing pulse power. However, this method collides with the requirement of having a very sensitive receiving circuit because a sensitive receiver shows normally a high susceptibility to high power level signals. However, the invention provides an improved opto-electronic receiving circuit preventing the receiver from being saturated and thermally imbalanced thus leading to a highly sensitive OTDR receiver that is tolerant against high power optical signals.
The invention can be partly or entirely embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit.